A Co Armagh scientist and his teenage son are sitting on a "green goldmine" after becoming the first farmers to grow a Japanese herb in Ireland.

Because they require very specific conditions, genuine wasabi plants are rarely seen outside Japan.

But Sean Kitson (51), who is originally from England, and his son Zak (17) have successfully cultivated the popular sushi accompaniment in a specially adapted polytunnel at their home in Tandragee.

"It's gone very well, almost too well," Sean said. "Wasabi is often referred to as 'green gold' so we are in a very good position. Our biggest challenge now is marketing."

But with only a handful of companies in Europe selling the scarce leafy plant that is normally found growing on stream beds in the mountain valleys of Shizuoka in the Land of the Rising Sun, he hopes his product will essentially sell itself.

"We are already selling to Zen restaurant in Belfast. We have had phone calls from Holland and from someone who wanted to buy the lot. There is huge demand out there," he said.

Their yield weighed in at 100kg, making it a bumper harvest, given that the 'king of herbs' is worth around £250 per kilo.

And with two acres of land to farm, the father-and-son business could rake in up to £2.5m every time they harvest their organically grown crop, which takes two years to mature.

"The battle is against the forces of nature, which is mainly slugs," Sean explained.

It was trainee chef Zak who came up with the idea after his father encouraged him to start a business to ensure he could be independent as an adult.

"Zak was diagnosed with multiple sclerosis when he was only seven. I felt it was important for him to do something like this so that he would be able to manage better when he goes through bad spells," his proud father explained.

"This is what he came up with, and thankfully it has worked out."

The medicinal plant has a reputation for its anti-bacterial and anti-inflammatory properties.

"There is also research indicating it has anti-cancer properties," Sean added.

"People in Japan rub it on their heads because it reportedly speeds up hair growth. As a scientist, I find this side of it fascinating."

The chemist, who works at Almac in Craigavon, spent countless hours trawling through research papers to ensure the notoriously difficult-to-grow plant would flourish.

"If it wasn't for my background in science I don't think we would have done this," he said.

The entrepreneurs now hope to become the main supplier of the herb to restaurants in Ireland and Britain.

But as cultivators of the sought-after plant, they could conquer the European market - and one day even export to Japan.

"A lot of chefs use alternative ingredients with a green dye because it is so hard to source and there's only 6,000 tonnes produced in the world," Sean said.

Zen owner Eddie Fung has given the local wasabi the thumbs-up.

"He said it's the real McCoy," Sean added.

Speaking on RTE's farming programme Ear To The Ground, which aired last night, Mr Fung said he had not been able to source wasabi in Ireland before now, and described Sean and Zak's product as "fantastic".

The Kitson family, including Sean's wife Olivera (51) and daughter Sofia (18), who also lend a helping hand, are now focused on ensuring another bumper harvest in 2020.

More information about the wasabi can be found online at www.wasabicrop.blog.